The present invention relates to a liquid crystal display device which is used in an optical display device, and more particularly, to a light-scattering type liquid crystal display device having multiple liquid crystal layers isolated by multiple electrical insulation layers.
In general, since a liquid crystal display device consumes a small amount of electricity due to its low driving voltage, such devices have recently undergone remarkable development and application fields thereof have also been extensively diversified. In a currently utilized liquid crystal display (LCD) device, since an active matrix type LCD device using a simple matrix or thin film transistor (TFT) uses a twisted nematic (TN) type or super twisted nematic (STN) type liquid crystal, at least a polarizer for controlling light, is required. However, while controlling the light, the polarized plate in the LCD intercepts more than 50% of the emitted light. Accordingly, efficiency in the use of light is severely curtailed.
Therefore, in order to obtain a picture having a desired brightness, a background light source having high brightness is required. Thus, in the case of a laptop wordprocessor or computer which uses a dry cell battery or an accumulative battery as a power supply, extended use is limited due to the excessive power consumption of the light source.
Meanwhile, in the general LCD device including the TN or STN type liquid crystal displays, since liquid crystal fills the space between two glass plates, a cell gap which is a light-controlled area is necessary for being strictly adjusted to form a uniform picture. Due to current technological limitations in the manufacturing of the glass plate, the super-enlargement of a liquid crystal display panel is difficult to achieve.
Taking into consideration the above-mentioned problems, in order to decrease the burden of cell gap adjustment, the polarized plate should not be used to increase efficiency in the use of light, and instead, a single base plate is used. Of course. LCD devices which do not use a polarized plate have been disclosed. Examples of such an LCD without polarized plate include a cholesteric nematic transition (CNT) type which uses a phase transition effect and a dynamic scattering mode (DSM) type which was devised during early LCD development. The DSM type liquid crystal display device exhibits a slow response time and cannot be made thin, so that it is no longer in common use.
Also, another example of an LCD not using a polarized plate to increase the efficiency of light is a polymer-dispersed liquid crystal display (PDLCD). However, since the PDLCD is made of a polymer material more than 50% of whose volume is light-transmitting, the scattering of light should be brought about efficiently to obtain a clear contrast ratio. There is a structural limitation in attaining these requirements in that the thickness of the liquid crystal layer should be at least 20 .mu.m.
The disclosure of an LCD which adopts an electrical field effect type liquid crystal having a new structure in which the above conventional problems of the LCD are considerably improved, was filed as U.S. patent application Ser. No. 08/058,712. A continuation-in-part application thereof for improved structure was filed on Aug. 24, 1993.
The above LCD has a fast driving speed and a high utilization efficiency of light, in which the liquid crystal layer provided between the opposing electrodes is isolated by a plurality of insulation layers to form a multi-layer structure, the polarized plate is not used and only a single sheet of a glass substrate is employed.
Hereinbelow, the structure of the reflective type liquid crystal display device proposed above U.S. patent application Ser. No. 08/058,712 and manufacturing method thereof will be described with reference to the accompanying FIGS. 1 through 14.
FIG. 1 is a schematic perspective view of the reflective type liquid crystal display device proposed above. Multiple liquid crystal layers 22 being the electrical field effect type are provided between opposing electrodes 10 and 18 on the substrate 16. The distance between liquid crystal layers 22 is maintained by columns 12. Insulation layers 20 for separating liquid crystal layer 22 into a plurality of layers are provided between liquid crystal layers 22. The mutual location of insulation layers 20 is fixed by columns 12 which are provided locally and liquid crystal injection holes 14 for locally injecting the liquid crystal are provided in the insulation layer 20. Here, the thickness each liquid crystal layer 22 which is partitioned from the liquid crystal layer 22 is less than or equal to 3 .mu.m and the thickness of each insulation layer 20 is preferably less than or equal to 5 .mu.m. The insulation layer 20 can be made of an epoxy resin material, or a metal oxide, particularly an aluminum oxide.
FIG. 2 is a partly extracted plan view of the liquid crystal display device of the FIG. 1, and FIG. 3 is a cross-sectional view along line A--A' of FIG. 2. Here, electrodes 18 are disposed oil a substrate 16 in a predetermined pattern. The electrodes 18 are covered with light-transmitting electric insulation resin layer 20 and multiple liquid crystal layers 22 and electric insulation layers 20 are laminated thereon. A reference numeral 10 denotes upper electrodes which oppose lower electrodes 18, 24 denotes photoresist layers remaining after the forming process, 26 denotes light-transmitting electric insulation layers, and 11 denotes light-shielding plates positioned above columns 12.
FIG. 4 is a cross-sectional view along line B--B' of FIG. 2. In FIG. 4, a reference numeral 14 represents liquid crystal injection holes, with like parts being designated by the same reference numerals used in FIG. 3.
The manufacturing method (steps A through G) of the liquid crystal display device having the above-mentioned process is explained with reference to the accompanied FIGS. 5 through 10.
Referring to FIG. 5, first electrodes of a predetermined pattern made of a conductive material such as an indium tin oxide (ITO) is formed on a black plastic substrate 16.
Referring to FIG. 6, insulation layers 20 made of a light-transmitting resin, for example, an epoxy resin, and dissolution layers 22a made of a material such as a polyvinyl alcohol (PVA) are alternately laminated on the whole upper surface of the substrate on which first electrodes 18 are formed by spin coating method or roll coating method. Thereafter, a second electrode 10 made of a conductive material such as ITO is formed on the uppermost layer among insulation layers 20 in a predetermined pattern.
Referring to FIG. 7, photoresist layers 24 having a pattern for columns to be described later are formed on the uppermost surface of the structure of FIG. 6.
Referring to FIG. 8, the portion not covered by the photoresists are plasma-etched to form inlet holes for forming columns 12. Then, the holes are filled with an epoxy resin. The epoxy resin is coated on the laminated layers to form columns 12 and a surface insulating layer 26.
Referring to FIG. 9, liquid crystal inlet holes 14 are formed by photo mask patterning and plasma etching. Here, water, acetone or alcohol is injected via the liquid crystal inlet holes 14 to thereby dissolve and remove the dissolution layers 22a. Accordingly, cavity portions 22b are provided between the epoxy resin layers 20. Each insulating layer 20 is supported by columns 12.
Referring to FIG. 10, after a semi-processed liquid crystal display device is dried, liquid crystal is coated on the whole surface thereof under vacuum to be injected to the cavity portions 22b via the injection holes 14 to thereby form liquid crystal layer 22.
Thereafter, after liquid crystal filling process is completed, in order to seal the liquid crystal, an electric insulation layer 26 made of epoxy resin is formed. Light-shielding plates 11 are formed on the columns 12 and injection holes 14,, which must be shielded from light in order to complete a liquid crystal display device.
In the aforementioned manufacturing method of the liquid crystal display device, a metal such as aluminum can be used as a material of the dissolution layer, instead the water-soluble PVA, and a light-transmitting electrically insulating resin including an acryl resin, semiconductor or metal oxide can be used as a material or the electric insulation layer, instead of the epoxy resin.
However, according to the above LCD manufacturing method, the following problems may result.
First, since considerable stress is concentrated on the central area of the light-transmitting electric insulation resin which is supported by the columns, it is difficult to maintain a constant gap size of the liquid crystal layers formed between the electrically insulating resins. Thus, a uniform picture is not obtained.
Next, when liquid crystal injection holes are formed as long inlet holes of about 10 .mu.m in length, a solvent for etching the dissolution layers has difficulty in penetrating into the narrow holes and, accordingly, a longer etching time becomes necessary. If the time for etching becomes longer, its contact time with the electrically insulating resin and etchant is also increased. Then, an undercut portion of the insulating resin may be formed. If the etching time is shortened to prevent the undercut formation, the dissolution layers are not completely removed.